Coke prevention in distillation of alkyl chlorides

ABSTRACT

THE FORMATION OF CARBON SOLIDS (COKE) DURING THE PHYSICAL PROCESSING OF ALKYL CHLORIDE-CONTAINING STREAMS IN FERROUS METAL APPARATUS SUCH AS FRACTIONATORS IS SUBSTANTIALLY REDUCED BY CARRYING OUT SUCH PROCESSING IN THE PRESENCE OF UP TO ABOUT 1% BY WEIGHT OF LOWER ALKYLATED AROMATICS; POLYMETHYLATED BENZENES AND METHYLATED NAPTHALENES ARE PREFERRED.

A ril 4, 1972 0. A. SCHEXNAYDER ETAL 3,654,093

COKE PREVENTION IN DISTILLATION OF ALKYL CHLORIDES Filed Sept. 11, 1969 DECENE HCI I7 22 M29 I2 25 DECANE l4 I9 27 CHLORINE ANTl-COKlNG I 3| AGENT 32 FIG. I

124 I I TRIDECENE us X HCI -I34 I22\ "2) TRIDECANE\ n4 CHLORINES ANTl-COKING AGENT INVENTORS:

o. A. SCHEXNAYDER 0 1. W000 THEIR ATTORNEY United'States Patent O Filed Sept. 11, 1969, Ser. No. 857,019 Int. Cl. B01d 3/34 U.S. Cl. 203-8 4 Claims ABSTRACT OF THE DISCLOSURE The formation of carbon solids (coke) during the physical processing of alkyl chloride-containing streams in ferrous metal apparatus such as fractionators is substantially reduced by carrying out such processing in the presence of up to about 1% by weight of lower alkylated aromatics; polymethylated benzenes and methylated napthalenes are preferred.

BACKGROUND OF THE INVENTION Alkyl chlorides are commercially important materials. Lower alkyl chlorides, such as methyl chloride, ethyl chloride and carbon tetrachloride are produced as end products (solvents and the like) and as chemical intermediates and higher alkyl chlorides, for instance C to C alkyl chlorides, are produced from alkanes as chemical intermediates for olefin preparation, alcohol productions, and amine production.

In practical scale operation, alkyl chlorides are generally produced from parafiins by one of a multiplicity of processes, such as thermal or catalytic vapor phase chlorination, catalytic oxychlorination, or liquid phase chlorinations, which produces a crude product containing the desired alkyl chlorides in admixture with unreacted paraflin, feedstock impurities, and by-products such as polychlorides. In such processes, it is normally desired to recover a specific alkyl chloride or a group of alkyl chlorides from the crude product mixture.

The separation of an admixture of purified alkyl chlorides from purified parafiins can be effected by distillation and like processes on a laboratory scale in glass or Monel apparatus with little or no difliculty. The practicalscale recovery of alkyl chlorides from typical crude mixtures, however, is far more difiicult. These recoveries necessitate processing such mixtures at elevated temperatures in equipment such as heat exchangers and fractionators, and the auxiliary piping associated therewith. Typically, these pieces of equipment are fabricated of ferrous metals or ferrous metal alloys. When impure alkyl chloride-containing mixtures are contacted with such ferrous metals, sludges of solid carbonaceous materials (coke) often tend to form. These solids build up in processing equipment to an extent that heat exchangers, fractionator kettles and plates, and the like become inoperably fouled. This necessitates frequent plant shutdowns and appreciable plant maintenance.

The causes of this carbon formation (coking) are not fully understood. It is known that ferrous metals, that is, iron, steel, and stainless steel, either in the form of metallic surfaces or as metal ions formed by attack of metals by the corrosive crude materials, catalyze the formation. It has further been found that the conversion of aromatic impurities in paraffin feedstocks is a major source of carbon solids.

An approach to solving this coke formation problem has been to add materials to process streams which inhibit coke formation. U.S. Pat. No. 1,971,318 issued on Aug.

ice

21, 1934 to L. C. Stewart et al. discloses the use of certain rosins, such as gum mastic, to inhibit the tendency of chlorinated hydrocarbon streams to decompose U.S. Pat. 2,543,575 issued on Feb. 27, 1951 to C. C. Harvey et a1. discloses the use of certain nitrogen-containing materials to prevent alkyl chloride decomposition during fractionation. More recently, U.S. Pat. 3,420,749 issued Jan. 7, 1969 to F. C. Dehn discloses the addition of organic esters of phosphoric acid and phosphorus acid to alkyl chloride streams to inhibit fouling and carbon formation.

STATEMENT OF THE INVENTION It has now been found that the propensity of alkyl chloride-containing mixtures to form solid carbonaceous materials during physical processing at elevated temperatures in the presence of ferrous metals or ferrous metal ions is reduced substantially when such mixtures are provided with lower alkylated aromatic hydrocarbons. Generally, addition of alkylated aromatic compounds of less that about 1% by weight are sufiicient to effect a substantial reduction of carbonaceous-solid formation (cok ing). This finding is especially surprising when considered in light of the coke-forming property of aromatics in general.

The invention will be further described with reference to the accompanying drawing wherein FIGS. 1 and 2 illustrate more or less diagrammatically two representative forms of apparatus in which the invention may be suitably practiced.

DETAILED DESCRIPTION OF THE INVENTION Alkyl aromatics utilized in accordance with this invention are alkylaromatics of 1 to 3 aromatic rings and, include monoalkylated and polyalkylated benzenes, naphthalenes, and anthracenes. Very suitable among these are alkyl benzenes, naphthalenes and anthracenes having from one to about four alkyl substituents each of which has from about one to about ten carbon atoms, such as for example, the xylenes, mesitylene, durenephenylethane, 4-methylphenylethane, 3,S-dimethylphenylethene, isobutylbenzene, the cymenes, l-methylnaphthalene, 2-methylnaphthalene, 1,2-dimethylnaphthalene, 1,S-dimethylnaphthalene-l,4,5, S-tetramethylnaphthalene, 1-methyl-3-ethylnaphthalene, lmethylanthracene, 2-methylanthracene, and 9-methylanthracene. Preferred alkylated aromatics according to the invention are alkylated benzenes and naphthalenes having from about one to about four alkyls of from one to four carbons each per molecule at least one of which is a methyl group. Alkyl aromatics which are especially preferred are alkylated benzenes having from two to four methyl groups as the sole alkyl substituents and alkylated naphthalenes having from one to four methyl groups as the sole substituents. Typical of these preferred methylated materials are the xylenes, mesitylene, l-methylnaphthalene, and Z-methylnaphthalene. Among the methylnaphthalenes, the l-methyl isomer is preferred.

The alkyl aromatic inhibitors according to the invention are very effectively employed as coke inhibitors for higher hydrocarbon chloride-containing mixtures, especially crude mixtures containing aliphatic hydrocarbon chlorides having from about 6 to about 20 carbon atoms per molecule. The inhibitors of the invention may suitably be added to liquid mixtures containing such materials prior to or during processing steps, such as distillation, to separate or purify such materials.

In a preferred application, inhibitors according to the invention are used to prevent coking during the separation and purification by distillation of aliphatic hydrocarbon chlorides having a substantial proportion of linear alkyl hydrocarbon chlorides which have from about 8 to 18 carbon atoms per alkyl group from crude mixtures comprising such alkyl chlorides, primarily-normal paraffins of from about 8 to 18 carbons, and minor amounts of aromatic and branched paraffinic impurities and chlorination byproducts. Such a mixture is typically the product of the chlorination of a primarily-normal C -C paraifin stream by liquid phase chlorination or other usual commercial chlorination processes. In such an application, addition of inhibitor to the crude mixture of from about 50 p.p.m. to about 500 p.p.m., by weight, very suitably reduces the formation of coke and minimizes the need to shut down continuous apparatus to remove fouling solids. When the preferred methylated aromatic inhibitors are used, additions of from about 100 p.p.m. to about 250 p.p.m. by weight are preferred. Larger amounts of inhibitor may be employed, if desired.

The coke inhibitors according to the invention when compared to the C C paraflins and alkyl chlorides, in clude materials having boiling points both higher and lower than the parafiins and alkyl chlorides being processed. When used to inhibit coking in distillation columns the inhibitors may be added to the chloride-containing stream at any point before it enters the column or may be added directly to the column. When a higher boiling inhibitor is added, it is generally preferred to add the inhibitor to the column at some point above the bottom to prevent coke buildup on the distillation trays, etc. When lower boiling inhibitors are employed, they may suitably be added directly to the column reboiler, their volatility permitting a carry-up of inhibitor into the distillation column trays. The use of higher-and-lower boiling inhibitors is further illustrated with reference to the accompanying drawing, FIG. 1 diagrammatically illustrating one form of apparatus suitable for using a higher-boling inhibitor according to the invention.

For purposes of illustration, the use of l-methylnaphthalene as inhibitor in the separation of n-decane from decyl chloride during a chlorination/dehydrochlorination process for preparing olefins is described.

To liquid-phase chlorination reactor 11 is continuously charged a hydrocarbon stream consisting essentially of decane and chlorine through lines 12 and 14 respectively. Hydrogen chloride is produced and is removed as a gas through line 15. A liquid reaction product containing about 75% decane, about 20% decyl chloride about decyl polychlorides and miscellaneous impurities, such as heavy condensed aromatics, is removed and passed through line 16 to approximately the center of ferrousmetal fractionator 17. Anti-coking agent (l-methylnaphthalene) is added in controlled amount to the fractionator feed stream via line 19 to prevent coking and fouling in such fractionator. A tops product consisting of decane is removed through line 20 and either discarded through valved line 21 or preferably recycled to reactor 11 through line 22. A bottoms product containing decyl chloride, polychlorides, and anti-coking agent is removed from fractionator 17 through line 24 to dehydrochlorinator 25 where it is reacted in the presence of a suitable catalyst to form hydrogen chloride which is removed via line 26. A liquid product containing decene, unreacted decyl chloride and polychlorides, l-methylnaphthalene and undetermined byproducts is removed through line 27 to fractionator 29 wherein a tops product, decene, is separated and removed through line 30 and a bottoms product consisting essentially of unreacted chlorides, heavy byproducts, and anti-coking agent is removed through line 31. A part of this bottoms is discarded through valved line 33 and optionally a portion is returned to dehydrochlo rinator 25 via valve line 32. If desired, anti-coking agent can be recovered by'means not shown from the bottoms product removed via line 33 and recycled to line 19.

FIG. 2 illustrates diagrammatically one form of apparatus suitable for using a lower-boiling inhibitor. For purposes of illustration, the use of paraxylene as inhibitor in the separation of tridecane from tridecyl chloride is described.

N-tridecane and liquid chlorine are continuously charged to chlorination reactor 111 through lines 112 and 114 respectively. Gaseous hydrogen chloride is removed via line 115. Liquid reaction product containing unreacted tridecane, tridecyl chloride, polychlorides, and impurities is removed and passed through line 116 to approximately the center of vacuum fractionator 117, which is constructed of ferrous metals. Lower-boiling anti-coking agent (paraxylene) is continuously charged to the reboiler of fractionator 117 through line 119 to prevent fouling of the reboiler and fractionator plates of fractionator 117. A tops product containing tridecane and a portion of the paraxylene is removed from fractionator 117 via line 120 and either discarded through valved line 121 or preferably returned to reactor 111 via line 122. The recycled, low-boiling alkyl aromatic inhibitor does not interfere with the chlorination reaction and is chlorinated to only a minor extent. Additional paraxylene is removed through vacuum line 124 and is recovered in cold trap 125 and either discarded via valved line 126 or preferably returned to fractionator 117 via valved line 127. Bottoms product from fractionator 117 containing tridecyl chloride and polychlorides and a very minor amount of paraxylene is removed through line 129 to dehydrochlorinator 130. Gaseous hydrogen chloride containing the remaining paraxylene is removed via line 131. If desired, paraxylene may be recovered by means not shown. Dehydrochlorination product is passed to fractionator 134 through line 132. A tops product consisting essentially of tridecene is removed through line 135 and a bottoms product, containing unreacted tridecyl chloride and polychlorides is removed through line 136 and either discarded through line 136 or preferably in part returned to reactor 130 through line 137.

The invention will be further described by the following examples.

EXAMPLE I A stream of C primarily straight chain paraffin was chlorinated in a steel reactor to produce a crude product containing about 75 by Weight unreacted C parafiin, about 20% C alkyl chloride and about 5% C alkylene dichlorides. To simulate on a laboratory scale the distillation which in a commercial process might normally follow such chlorination to separate unreacted parafiin for recycle, a sample of this material was placed in an Erlenmeyer flask equipped with a reflux condenser. This material was heated at C. under a nitrogen atmosphere for one hour. The formation of coke-like carbonaceous fines was noted. The sample was analyzed following the one hour heating. 30 p.p.m. by weight of carbon solids had been formed by the heating. The sample contained 0.1 p.p.m. by weight ferrous iron and 0.3 p.p.m. ferric iron, apparently from the walls of the steel reactor in which the feed stock was prepared.

EXAMPLE II The experiment of Example I was repeated with one variation, 100 p.p.m. by weight of l-methylnaphthalene was added to the crude chlorinator product prior to heating. Analyses following the one hour heating at 195 C. indicated that only 2 p.p.m. by weight of carbon solids had been formed. The iron content of the sample following heating was 0.3 ppm. ferrous and 0.01 p.p.m. ferric iron.

EXAMPLE III The experiment of Example I was repeated several times using the iron-containing chlorinated parafiin feedstock of Example I. Varying amounts of several alkyl aromatics were added to inhibit the formation of carbonaceous fines (coke). The results of these experiments are given in the accompanying table.

Concentration carbonaceous We claim as our invention:

1. In a method for distilling a mixture containing alkyl chlorides at an elevated temperature in the presence of ferrous metals or ferrous metal ions with the formation of undesirable carbonaceous solids the improvement of reducing the formation of carbonaceous solids by carrying out the processing in the presence of about 25 p.p,m. to about 1% by weight of added alkylated naphthalenes having from 1 to 4 methyl groups as the sole alkyl substituen-ts.

2. The method in accordance with claim 1 wherein the mixture containing alkyl chlorides is a crude paraffin chlorination product comprising paraflins of from 6 to 20 carbons and alkyl chlorides of from 6 to 20 carbons.

3. The method in accordance with claim 2 wherein the amount of added alkylated naphthalene is from about 50 ppm. to about 500 ppm. by weight.

4. In a method for distilling a mixture containing alkyl chlorides at an elevated temperature in the presence of ferrous metals or ferrous metal ions with the formation of undesirable carbonaceous solids the improvement of reducing the formation of carbonaceous solids by carrying out the processing in the presence of about 25 p.p.m. to about 1% by weight of added l-methylnaphthalene.

References Cited UNITED STATES PATENTS 2,868,851 1/1959 Skeeters 260-6525 1,917,073 7/1933 Stewart et a1 260-6525 2,580,730 1/1952 Church et a1 260-6525 2,660,591 11/1953 Calingaert 203-6 FOREIGN PATENTS 579,470 7/ 1959 Canada.

NORMAN YUDKOFF, Primary Examiner D. EDWARDS, Assistant Examiner U.S. Cl. X.R. 

